In micromechanical sensors, of which a silicon microphone is an example, there are often air-filled cavities having very small dimensions. In a microphone, this is, for example, the air-filled sensor capacity consisting of a sensitive membrane and a rigid counter electrode. Due to the small air volume, the entrapped air applies a strong restoring force on the sensor membrane. This rigidity of the entrapped air lowers the sensitivity of the sensor.
It is known to provide discharge possibilities for the air, wherein this, in silicon microphones, takes place by perforation of the counter electrode. By means of such a perforation, the air can escape from the capacitor gap, i.e. the cavity between the sensitive membrane and the rigid counter electrode.
A large number of microphones and micromechanically manufactured microphones are known from the prior art.
Commercial electret microphones comprise geometries in an order of magnitude in which the rigidity of the air cushion is negligible. These microphones do not have the advantages of a temperature-stable silicon microphone when produced in large numbers.
In micromechanically manufactured microphones, such ones having electroplated counter electrodes are known in which the counter electrode is finally electroplated on the microphone chip. As regards such microphones, reference is, for example, made to Kabir et al., High sensitivity acoustic transducers with p+membranes and gold black-plate, Sensors and Actuators 78 (1999), pp. 138–142; and J. Bergqvist, J. Gobet, Capacitive Microphone with surface micromachined backplate using electroplating technology, Journal of Microelectromechanical Systems, Vol. 3, No. 2, 1994. In methods of manufacturing such microphones, the size of holes in the counter electrode can be selected such that the acoustic resistance is very small and does not influence the microphone sensitivity. The complicated process of electroplating is, however, of disadvantage.
Two-chip microphones are also known from the prior art, in which the membrane and the counter electrode are manufactured on respective separate wafers. The microphone capacity is then obtained by bonding the two wafers. As regards such a technology, reference is made to W. Kühnel, Kapazitive Silizium-Mikrofone (Capacitive Silicon Microphones), series 10, Informatik/Kommunikationstechnik, No. 202, Fortschrittsberichte, VDI, VDI-Verlag, 1992, Dissertation; J. Bergqvist, Finite-element modelling and characterization of a silicon condenser microphone with highly perforated backplate, Sensors and Actuators 39 (1993), pp. 191–200; and T. Bourouina et al., A new condenser microphone with a p+ silicon membrane, Sensors and Actuators A, 1992, pp. 149–152. It is, as far as technology is concerned, also possible in this type of microphone to choose adequately large diameters for the holes in the counter electrode. For reasons of cost, however, one-chip solutions are preferred. In addition, the calibration of the two wafers is problematic in two-chip microphones.
In one-chip microphones mentioned before, the counter electrode is manufactured in an integrated way, i.e. only one wafer is required. The counter electrode is made of a silicon substrate and is formed by means of deposition or epitaxy. Examples of such one-chip microphones are described in Kovacs et al., Fabrication of singe-chip polysilicon condenser structures for microphone applications, J. Micromech. Microeng. 5 (1995), pp. 86–90; and Füldner et al., Silicon microphone with high sensitivity diaphragm using SOI substrate, Proceedings Eurosensors XIV, 1999, pp. 217–220. In the manufacturing processes for these one-chip microphones, it is required or of advantage to close the holes in the counter electrode again for the following processing in order to smooth the topology. In the well-known micromechanically machined microphones described above, the perforation openings in the counter electrodes have a squared or circular cross-sectional form.
A manufacturing process for such one-chip microphones is known from WO 00/09440. In this method for manufacturing, the perforation openings are formed at first in an epitaxial layer formed on a wafer. Subsequently, an oxide deposition on the front side of the epitaxy layer is performed so that the perforation openings are closed on the one hand and a spacing layer, the thickness of which defines the future gap between the membrane and the counter electrode is formed on the other hand. A silicon membrane having the required thickness is then deposited on this layer. After the required processing of the electronic elements, the wafer is etched from the backside down to the epitaxy layers in the region of the perforation openings. Subsequently, etching of the oxide from the backside takes place for opening the perforation openings and the cavity between the membrane and the counter electrode. A part of the sacrificial layer between the membrane and the epitaxy layer thus remains as a spacing layer between the membrane and the counter electrode.
A method of manufacturing a one-chip microphone is known from DE 19741046 C1, in which the counter electrode is patterned so to speak in a final manufacturing step after producing the membrane. Thus, it is possible in this method to produce holes having a diameter of about 25 μm to 50 μm or squares having an edge length of about 25 μm as perforation openings. In addition, this text teaches providing perforation openings in the counter electrode which have the form of a rectangle which, with its longitudinal sides, extends over almost the entire edge length of the squared counter electrode and the width of which corresponds to the edge length of the squares indicated above.
Finally, capacitive transducers are known from U.S. Pat. No. 5,870,482, in which border regions of a mounted membrane, together with a counter electrode, serve as capacitive receptors. In one example, a round membrane mounted in its middle section is provided, while the outer border region, together with a counter electrode spaced apart between 1 μm and 4 μm, forms a capacitor. 14 μm slots having a spacing of 24 μm are provided in the counter electrode.